Seepage Losses in Reservoirs

Seepage losses also diminish the volume of water in reservoirs. The magnitude of these losses is related to the permeabilities of the enclosing bed rocks,

Table 5.6

Reduction in Reservoir Capacity due to Siltation

Table 5.6

Reduction in Reservoir Capacity due to Siltation

Annual reduction in

Reservoir

Period

capacity (%/yr)

High Aswan Dam, Nile River, Egypt

1967-1991

0.05

Sampling of U.S. reservoirs

19.7 yr period

0.2

Lake Mead, Colorado River

1935-1949

0.3

Bhakra Lake, Sutlej River, India

1959-1969

0.5

Sennar (Makwar Reservoir), Blue Nile, Sudan

1925-1986

0.6

Imagi Reservoir, Kenya

0.8

Roseires Reservoir, Blue Nile, Sudan

1966-1981

1.3

Bhumipol Reservoir, Thailand

1.5

Tarbela Dam, Indus River, Pakistan

1974-1990

2.0

Khashm-el-Girba Reservoir, Atbara River, Sudan

1965-1990

2.2

Sanmexia Dam, Yellow River, China

1960-1990

2.2

Nizam Sagar Dam, Manjira, India

1931-1961

2.2

Shah-Banou Farah Reservoir, Iran

1961-1990

2.4

Estimated global average rate

1%

Data sources: Shahin, 1993; Rao and Palta, 1973; Dendy et al., 1973; Smith et al, 1960; Douglas, 1990; James and Kiersch, 1988.

Data sources: Shahin, 1993; Rao and Palta, 1973; Dendy et al., 1973; Smith et al, 1960; Douglas, 1990; James and Kiersch, 1988.

geologic structures, such as fractures or faults, and the height of the water table (James and Kiersch, 1988). Seepage losses occur where solutions can migrate away from the sides or base of the reservoir. These occur along fractures, faults, solution cavities or sinkholes in carbonate rocks (Moneymaker, 1969), unconsolidated and/or permeable sedimentary rocks (Gardner, 1969), and lava tunnels (Monahan, 1969).

Very few data exist on seepage losses. Lake Nasser may be losing up to —0.6% of capacity per year (Wafa and Labib, 1973). However, storage of water in dry rock pore volume may ultimately reach up to 29% of maximum reservoir capacity at Lake Nasser. Lake Mead lost —1.7% of its initial capacity per year in the 1940s (Smith et al., 1960). Annual seepage losses for several small mid-Western reservoirs range from 0.1 to 39% of capacity (Gardner, 1969). Unfavorable geologic conditions leading to severe reservoir leakage have resulted in reservoir abandonment and even dam failure, in several extreme cases (Kiersch, 1958).

Although seepage data are fragmentary, an annual average loss has been estimated at up to —5% of reservoir volume (Gleick, 1992). In the absence of additional data, we assume an average annual loss of 5 ± 0.5% of reservoir volume for reservoirs built up to the 1990s. This amounts to 203-293 km3/yr, or around 0.56-0.81 mm/yr withheld from SLR (Table 5.4).

Percolation beneath reservoirs recharges aquifers, which ultimately increases the groundwater discharge to the sea. However, because the average global residence time for groundwater in aquifers has been estimated at around

330 years (L'vovich, 1979) and possibly even thousands of years or more (Jones, 1997), this effect is probably relatively small over the short (<100 year) periods considered here and has therefore not been taken into account. Nevertheless, some of this seepage may actually enter groundwater discharge and/or river runoff, even within a few years, thereby not adding to storage of water on land. Thus the estimated water sequestration from this source could be high.

5.3.2 Irrigation

Irrigation is a major consumer of worldwide freshwater resources, accounting for around 39% of all freshwater use in the United States (Solley et al., 1998) and 69% internationally (WRI, 1998). As in the case of reservoirs, losses of irrigation water (I) to deep seepage and evaporation also lessen the anthropogenic input to SLR. In 1996, 263 million ha were under irrigation, with 70% of the total in Asia, 9.5% in Europe, 5% in Africa, and 15% in North, Central, and South America (FAO, 1997).5 The average annual use of water in irrigation varies widely, ranging from around 12,000 to 15,000 m3/ha in North Africa to less than 4000 m3/ha in several more water-efficient western European countries (Framji et al., 1981). The area-weighted global average water use in irrigation is approximately 9450 m3/ha. This figure multiplied by the global area under irrigation gives a volume of 2484 km3. Conveyance losses in open canals and ditches may increase water losses by a factor of 1.3 (Postel, 1989). Thus, the total comes to 3229 km3.

A substantial fraction of the water applied in irrigation is used consumptively, that is, it is taken up and evapotranspired by crops, and thus does not return to streamflow (at least not in the short run; a small, but not well-known quantity could enter via underground or surface discharge). Globally, the consumptive use of water in irrigation is around 78 ±6% of the water withdrawn (Shiklomanov, 1997). It varies regionally between 61% in North America and 82% in Asia. Using an average value of 78% combined with the above estimate of water used in irrigation gives 2519 km3/yr, with a range between 2325 and 2712 km3/yr. A small, as yet unquantified amount of water from évapotranspiration remains in the atmosphere. An accurate assessment of this fraction is needed; if we assume that up to 2% of the evaporated water remains in the atmosphere as added vapor, then the amount of SLR withheld through évapotranspiration would be as much as 0.12 mm/yr (0.10-0.15 mm/ yr) in the 1990s (Table 5.4).

Irrigation water not lost in évapotranspiration (16-28%) infiltrates into the soil (equivalent to 516.6-904.1 km3). If around 5 ±0.5% of this water is assumed to percolate to depth, as in the case of reservoirs, this yields 23-50 km3. In addition up to 98% of the irrigation water lost through évapotranspiration eventually reprecipitates. If again 5 ±0.5% of this recycled water infiltrates

5 The 1996 figure for the United States is missing, so the 1995 value was added to the global total.

at depth, this removes another 103-146 km3. A total of 143-175 km3 could therefore be sequestered at depth, corresponding to 0.40-0.49 mm/yr withheld from SLR (Table 5.4). As indicated above, because of seepage, these estimates are upper bounds.

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